During the manufacture of a semiconductor device, various processes such as film deposition, surface modification, oxidation/diffusion, and etching, are conducted upon the semiconductor wafer that is a substrate to undergo processing. The etching process most commonly uses plasma etching in which the wafer is etched with a resist film of a desired pattern as a mask by using a plasma to obtain higher processing accuracy.
In recent years, a tungsten-containing film such as a tungsten (W) film, tungsten silicide (WSi) film, or tungsten nitride (WN) film, has been most commonly used to form the gate electrode of a MOS-type semiconductor. For the formation of such a gate electrode, a semiconductor wafer is first prepared that has a structure in which a gate oxide film, a polysilicon film, and a tungsten-containing film are formed sequentially over a silicon substrate first and then a patterned etching mask is formed on the tungsten-containing film. Next, the tungsten-containing film on the semiconductor wafer is etched to form the gate electrode.
The etching of the tungsten-containing film has traditionally used a chlorine-based gas such as Cl2, HCl, or SiCl4, or a fluorine-based gas such as CF4 or SF6. For enhanced selectivity with respect to an under-layer, O2 has also been added to these gases in some cases, as described in, for example, Japanese Laid-Open Patent Application Publications JP-A-2004-39935 and JP-A-2000-235970.
An etched-surface geometry/morphology and selectivity with respect to the under-layer become important factors in forming the gate electrode by etching the tungsten-containing film. An even better etched-surface geometry/morphology and an even higher selectivity with respect to the under-layer are being called for with the growing demands for the further microstructuring and higher-density mounting of semiconductor devices in recent years.
However, chlorine-based gases are low in reactivity, and it is therefore inconvenient to use such a gas, since the temperature of the semiconductor wafer must be correspondingly increased to obtain an appropriate etched-surface geometry/morphology (vertical profile geometry/morphology). In contrast, the use of a fluorine-based gas provides an excellent geometry, but results in the insufficiency of an etching selectivity with respect to the under-layer. For these reasons, it is difficult with conventional technology to simultaneously obtain the appropriate etched-surface geometry/morphology and high selectivity with respect to the under-layer.